<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
      .box1{
          width: 500px;
          height: 500px;
          background-color: red;
          margin: 0 auto;
          position: relative;
      }
      .child{
          width: 100px;
          height: 100px;
          background-color: yellow;
          position: absolute;
          top: 50%;
          left: 50%;
          margin-top: -50px;
          margin-left: -50px;
      }
    </style>
</head>

<body>
    <div class="box1">
        <div class="child"></div>
    </div>
    
</body>

</html>